A comparison of surface-enhanced infrared and surface-enhanced Raman spectra of pyrazine adsorbed on polycrystalline gold electrodes

The adsorption of pyrazine at a polycrystalline Au film electrode has been investigated using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), chronocoulometry, and ac impedance. Combining the SEIRA data and the thermodynamic data (the surface charge density of the electrode and the relative Gibbs surface excess of pyrazine), pyrazine was found to adsorb on the surface with a vertical end-on configuration via one N atom. The SEIRA spectra were compared with published surface-enhanced Raman (SER) spectra of pyrazine in order to clarify the reason for the breakdown of the Raman selection rule in the SER spectra. The activation of originally Raman-forbidden modes in the SER spectra is well explained by a photo-driven charge-transfer mechanism. The charge-transfer is deduced to be from filled metal states near the Fermi level to the first and second excited states of pyrazine. It is emphasized that the comparative SEIRA and SER studies are important for a better understanding of the electrochemical interface.     

Vibrational spectroscopic studies of three dimensional host lattices formed by pyrazine bridges between tetracyanonickelate layers

Three dimensional host lattices have been developed by forming bridges with bidentate pyrazine molecules between adjacent tetracyanonickelate polymeric layers of Ni(II) or Cd(II). The Fourier-transform IR and Raman spectra (4000-200 cm^–1) of the compounds with the general formula M(pyz)Ni(CN)₄, (where M = Ni or Cd) are reported. These host lattices can include benzene molecules but it is found that aniline molecules cannot be included in these structures. They, however, form complexes with the formula M(an)₂Ni(CN)₄, by replacing pyrazine ligands. A monodentate pyrazine complex of Cd(II) with the formula Cd(pyz)₂Ni(CN)₄ has also been prepared.     

Vibrational and theoretical study of the 2',6'-dimethoxyflavone cis-formic acid inclusion compound

The FT-infrared and Raman microscopy spectra of the 2',6'-dimethoxyflavone and its 1:1 complex with formic acid in solid state have been recorded and analysed. Some vibrational components appear as specific to the cis-rotamer of formic acid in the crystalline sample, especially the CH group stretching vibration feature. The broad and intense infrared absorption observed in the range 3400-1900 cm(-1) and assigned to the hydrogen bonded OH group stretching vibration exhibits the characteristic ABC structure of strong hydrogen bonded complexes. This ABC pattern corroborates previous X-ray crystallographic data showing that cis-formic acid is strongly hydrogen bonded to the flavonic compound. The inclusion complex is quite unstable and the infrared spectrum clearly shows that formic acid disappears after a period of a few months. In order to get some information on the stability criterions of the intermolecular hydrogen bonded complex, semiempirical AM1 calculations have been investigated. The comparison of the calculated heats of complexation (deltacH) for chelates involving the cis- and trans-conformers of formic acid suggests that the reaction of hydrogen bonding complexation with the cis-rotamer is surely favoured.     

Vibronic coupling and Raman intensities of pyrazine

The fluorescence spectrum of pyrazine vapour and the Raman spectra of liquid pyrazine excited at various wavelengths were observed. The appearance of the long progression of the hydrogen bending vibration ν₅ (b_2g) in the fluorescence spectrum and a remarkable preresonance effect of the Raman line of the same vibration confirm the theoretical prediction given by Albrecht.     

The ionization of trichloroacetic acid and evidence for an unusual type of ion pairing

Osmotic and activity coefficients are reported for aqueous solutions of the lithium and potassium salts of iodic acid and trichloroacetic acid. The degree of ionization of the parent acids at various concentrations was estimated from ion exchange measurements, and these values were compared with those obtained from Raman measurements. It was concluded that one obtains comparable values of α, the degree of dissociation, of iodic acid whether these are estimated from hydrogen ion concentrations or anion concentrations. Lack of agreement between like measurements of α for trichloroacetic acid leads to the postulation of an unusual ion pair in which the proton is strongly associated with the chlorine end of the anion. A similar type of ion pairing is suggested in solutions of the aromatic sulfonic acids.     

Melting curve and high-pressure chemistry of formic acid to 8 GPa and 600 K

We have determined the melting temperature of formic acid (HCOOH) as a function of pressure to 8.5 GPa using infrared absorption spectroscopy, Raman spectroscopy and visual observation of samples in a resistively heated diamond-anvil cell. The experimentally determined incongruent melting curve compares favorably with a two-phase thermodynamic model. Decomposition reactions were observed above the melting temperature up to a pressure of 6.5 GPa, with principal products being CO2, H2O, and CO. At pressures above 6.5 GPa, decomposition led to reaction products that could be quenched as solids to zero pressure, and infrared and Raman spectra indicate that pressure leads to the presence of sp3 carbon-carbon bonding in these reaction products.     

Carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine: synthesis, structure, hydrogen bond and metal directed self-assembly

Cyclooligomerization of 2,6-dichloropyrazine 4 and benzyl 2,3-dihydroxybenzoate 5 under microwave irradiation resulted in a racemic pair of ester functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 6, which was further transformed to the corresponding racemic carboxylic acid functionalized ortho-linked oxacalix[2]benzene[2]pyrazine 3. Both enantiomers of 3 adopt 1,3-alternate conformations with their two carboxylic acid groups pointing to opposite directions in the solid state. Enantiomers of 3 form a step-like one-dimensional supramolecular polymer via intermolecular hydrogen bond interactions between the carboxylic acids for crystals obtained in methanol. No hydrogen bonds were formed between the carboxylic acids for crystals of 3 obtained in pyridine and aqueous guanidine solutions; instead, intermolecular hydrogen bonds between the carboxylic acid groups of 3 and pyridine, as well as guanidinium ions were formed. Under metal-mediated self-assembly conditions, the pyrazinyl nitrogen atoms in 3 interacted with transition metal ions, such as Ag(I), Cu(II) and Zn(II), and resulted in the formation of four new metal-containing supramolecular complexes. Metallomacrocycles 7, 8 and 9 were formed by reactions of 3 with Ag(I) or Cu(II) ions by bridging two ligands 3 in the equatorial region via M-N coordination bonds. A one-dimensional coordination polymer 10 was generated by reaction between ligand 3 and Zn(II) ions, and a cage-based structure is presented in 10 by bridging of the cyclophane units by Zn(2+) ions via Zn-N and Zn-O bonds.     

Infrared spectroscopy of acetic acid and formic acid aerosols: pure and compound acid/ice particles

Acetic acid aerosol particles, formic acid aerosol particles and mixed acid/ice particles were generated in a collisional cooling cell at a temperature of 78 K and investigated using in situ rapid scan Fourier transform infrared spectroscopy. The infrared spectra reveal that the internal structure of the particles critically depends on the particle formation conditions and, especially for the mixed particles, on the composition. The acetic acid particles are likely to have only a partially crystalline structure whereas the formic acid particles are likely to have an overall crystalline structure. The existence of acid in the mixed acid/ice particles prevents the ice from crystallization even at low acid concentrations (less than 10%). Mid-infrared refractive index data were derived from the different particle spectra, which can be helpful for remote sensing of such systems.     

Transition dipole—dipole coupling interactions in the B form of stearic acid single crystals

Raman spectra of the B form of stearic acid single crystals CH₃(CH₂)₁₆COOH have been recorded for various polarization geometries. Infrared spectra were obtained for the B powder form. A normal coordinate treatment of the crystal has been performed. The group coordinate force field describing the intramolecular potential, a hydroen-bond potential and intermolecular non-bonded atom—atom interactions were used. In addition transition dipole—dipole coupling interactions between the CO bonds in the unit cell were needed in order to explain the observed infrared and Raman bands, contrary to some previous observations concerning the existence of two different conformations of this dimeric carboxylic acid.     

Excitation wavelength dependent surface-enhanced Raman spectra of a dipping film of azobenzene-containing long-chain fatty acid on a silver mirror

Surface-enhanced Raman scattering (SERS) of dipping films of azobenzene-containing long-chain fatty acids, nAmH (n=8, 12, m=3, 5), on silver mirrors measured with a wide range of excitation wavelengths in the 457.9-1064 nm region is reported. The obtained Raman spectra show great SERS effect even with the 1064 nm excitation, and the excitation with 457.9, 476.5, and 488.0 nm gives surface-enhanced resonance Raman scattering (SERRS) due to the resonance effect of the symmetry-forbidden n-pi* transition of the azo group. Of particular note in the present study is that the SERS spectra with the excitation in the 532-1064 nm region yield Raman bands whose frequencies are almost identical to those bands in Raman spectra of nAmH in solid state while the SERRS spectra with the excitation in the 457.9-514.5 nm region show not only a set of bands which correspond to those of nAmH in the solid state but also a set of bands whose frequencies show a significant shift from those of the bands of nAmH in the solid state. These observations lead us to conclude that there are two kinds of molecular aggregates in the dipping films of azobenzene-containing long-chain fatty acid in which azobenzene moieties are condensed to form small bundles.     

Direct visual evidence for chemical mechanism of SERRS of pyrazine adsorbed on Ag nanoparticle via charge transfer

In this paper, the chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of pyrazine adsorbed on Ag nanoparticles through charge transfer was experimentally and theoretically investigated. Based on the calculations by density functional theory (DFT) and time-dependent DFT (TD-DFT), we theoretically analyzed the absorption spectra and SERS spectrum of the S-complex of pyrazine–Ag₂₀. The charge transfer in the process of resonant electronic transitions between adsorbed molecule and metal cluster can be visualized by the method of charge difference density. It is a direct evidence for the chemical enhancement mechanism of SERRS of pyrazine molecule adsorbed on Ag nanoparticle via charge transfer between molecule and metal. Additionally, the intracluster charge redistribution was also considered as an evidence for the electromagnetic enhancement. By comparing the experimental and theoretical results, it was demonstrated that the SERRS of the pyrazine molecule absorbed on silver clusters in different incident wavelength regions is dominated by different enhancement mechanisms via the chemical and electromagnetic enhancements.     

Orientation change of adsorbed pyrazine on roughened rhodium electrodes as probed by surface-enhanced Raman spectroscopy

A surface-enhanced Raman spectroscopic (SERS) study of pyrazine adsorbed on roughened Rh electrodes was performed. Potential and concentration effects on the adsorption behavior of pyrazine were investigated. The SER spectra display four pairs of overlapping bands with the relative intensity of each pair being highly potential dependent, which has not been observed on other metals. The orientation change of the adsorbed pyrazine from the end-on to N/pi bonded edge-on configuration is proposed to account for this potential-dependent relative intensity change. This hypothesis is further supported by the SERS results obtained at different pyrazine concentrations. In conjunction with the orientation effect, the interaction of Rh with hydrogen and oxygen generated at different potentials has a great influence on the adsorption configuration of pyrazine.     

Amino acid contents along the visual and equatorial axes of a pig lens by Raman spectroscopy

Using near infrared Raman microspectroscopy with laser light of 830 nm, the distribution of amino acids along the visual and equatorial axes of a normal pig lens was studied. The classification of pig lens Raman spectra in these axes was performed using principal component analysis and linear discriminant analysis. The analysis of the scattered light selectively collected from point to point, along the visual axis, indicated that the tyrosine and tryptophan increases and then, at approximately 4 mm position, decreases. Moreover, in the equatorial plane, the nuclear part has the highest concentration of these amino acids. However, the phenylalanine content increases from anterior to posterior cortex of the lens as long as in the equatorial axis it slightly increases and then at approximately 2-2.3 mm position, decreases. The changes in amino acid conformation along the visual axis, similarly to the changes in protein conformation, may explain the refractive gradient of the lens.     

Hydrothermal syntheses, structures, and properties of [Cu3Cl2CN(pyrazine)] and copper(I) halide pyrazine polymers

Crystals of copper halide and pseudohalide compounds with pyrazine are synthesized under hydrothermal conditions. The title compound, [Cu3Cl2CNPz] (1) (Pz = pyrazine), is a new copper compound exhibiting an unusual -(Cu3Cl2)- polymeric stair structural motif and three-coordinate cyanide. Compound 1 crystallizes in the monoclinic space group P2(1)/m, with a = 3.6530(7) A, b = 17.160(3) A, c = 6.9800(14) A, beta = 90.58(3) degrees, and Z = 2. In addition, the series of complexes [Cu2X2Pz] for X = Cl (2), Br (3), and I (4) are also crystallized under hydrothermal conditions. The inorganic polymer [Cu2Br2Pz] (3) belongs to the triclinic space group P1, with a = 6.9671(14) A, b = 7.849(2) A, c = 8.099(2) A, alpha = 71.69(3) degrees, beta = 70.71(3) degrees, gamma = 85.43(3) degrees, and Z = 2. The structure of 3, is similar to the recently reported structure for [Cu2Cl2Pz] (2) (Kawata, S.; Kitagawa, S.; Kumagai, H.; Iwabuchi, S.; Katada, M. Inorg. Chim. Acta 1998, 267, 143). The third member of the series, [Cu2I2Pz], is found to be isostructural on the basis of X-ray powder diffraction results. The lattice parameters are refined from indexed reflections to a = 7.115(10) A, b = 8.321(19) A, c = 8.378(17) A, alpha = 71.1(3) degrees, beta = 67.3(1) degrees, and gamma = 83.0(2) degrees. Electronic spectra show that compounds 1-4 have optical band gaps in the range 2.2-2.4 eV. The infrared and Raman spectra as well as the thermal properties of all compounds are presented.     

PMR spectra of protonated pyrazine and quinoxaline N-oxides

The dependence of the chemical shifts of the protons of quinoxaline and quinoxaline and pyrazine N-oxides and N,N-dioxides on the acid concentration was studied. The changes in the PMR spectra on passing from the neutral bases to the monocations were examined. The first protonation of pyrazine and quinoxaline N-oxides occurs at the N₄ nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 846–852, June, 1971.     

A comparative vibrational and NMR study of cis-cinnamic acid polymorphs and trans-cinnamic acid

The IR and Raman spectra of the two polymorphic forms (58 degree- and 68 degree-forms) of cis-cinnamic acid were measured, and the spectral differences discussed on the basis of the crystal structures of the two forms. The IR bands related to the COOH group differ in the frequencies and band shape, reflecting differences in the hydrogen bonding between the two modifications. These spectra were compared with those of trans-cinnamic acid. The IR, Raman, and NMR spectra of the isotopic compounds, including the deuterated and 13C analogs of the cis and trans acids, were also recorded in the solid state and in solution to confirm the spectral assignments.     

Ag(I) coordination polymer with pyrazine sulfonic acid: synthesis,crystal structure,and fluorescence property

A one-dimensional rigid, linear Ag(I) coordination polymer with pyrazine sulfonic acid, [Ag₂L₂?·?H₂O] _n (L?=?pyrazine sulfonic acid) has been synthesized and characterized by elemental analyses, infrared spectroscopy, thermogravimetric analysis, and X-ray crystallography. The structure determination shows Ag(I) centers linked by bridging pyrazine ligands to give a double-chain structure sustained by weak Ag-O and Ag-N interactions. The sulfonate and pyrazine are all involved in hydrogen bonding interaction. The interactions combine with weak π-π stacking to play deciding roles defining the supramolecular structures. A powder X-ray diffraction study implies good phase purity. Solid-state fluorescence quenching and thermal stability are also discussed.     

Intermolecular one-pot cyclization of formyl-pyrroles of amino acid esters with norephedrine: stereoselective routes to new tricyclic pyrrole–pyrazine–oxazole fused structures

The treatment of esters of amino acids with dimethoxytetrahydrofuran furnished pyrrole derivatives of amino acid esters. The Wilsmeier–Haack formylation followed by the reaction of the formylated pyrroles with norephedrine afforded a selective formation of the tricyclic pyrrole–pyrazine–oxazole fused structures in one step via the formation of an oxazoline structure and intramolecular lactam formation. Pyrrole–pyrazine–oxazole fused structures were achieved in good yields. The cyclization reaction for the formation of an oxazole ring worked selectively to form only one stereoisomer. The configuration of the newly generated stereogenic center in the oxazole ring is dependent on the stereogenic centers of norephedrine.     

Vibrational spectroscopic study on the quantum chemical model and the X-ray structure of gallic acid,solvent effect on the structure and spectra

Infrared, Raman and far-IR spectra of gallic acid were recorded both in crystalline and in its dry form. Solvent effect of water was studied. The molecular and crystal structures were determined by single crystal X-ray diffraction. A complex system of intermolecular interactions was revealed. Optimized geometries, vibrational frequencies and infrared intensities were calculated utilizing the post-HF DFT method with the Becke3LYP functional and the 6-31G* basis set. Normal coordinate analysis was carried out. The results of the calculations were applied to simulate the infrared and Raman spectra and the full assignment of the acquired spectra is presented.     

Indium(III) one-dimensional polymers with pyrazine and pyrazine, 2-carboxylic acid as the spacer molecules

The reactions of indium(III) chloride tetrahydrate with pyrazine (C₄H₄N₂) and pyrazine, 2-carboxylic acid afford two polymeric frameworks, the structures of which were characterized in the solid state by single crystal analysis. The former is a one-dimensional infinite structure interlinked by the pyrazine spacer, while the latter is a one-dimensional ‘zigzag’ polymeric structure. A dimeric indium(III) pyrazine complex is also reported.